Endogenous opioids modulate neurotransmission through a number of different sensory and motor systems. The present proposal is concerned in general with the modulation of sensory transmission and in particular with opioid modulation of nociceptive sensory systems at the spinal level. The intent of this proposal is to utilize recently developed antibodies to examine in detail the localization of opioid receptors in relation to different functional classes of primary afferent neurons in rats and monkeys. In addition, experiments will determine the spatial relationships between opioid receptors, opioids, and functionally and anatomically identified spinal neurons. In making this proposal, the following specific aims will be addressed: 1. To determine in rats and monkeys what functional classes of primary afferent neurons express opioid receptors. 2. To determine in rats the anatomical and spatial relationships between opioid receptor-containing neuronal elements and retrogradely labeled spinothalamic tract neurons. 3. To determine in monkeys the anatomical and spatial relationships between opioid receptors and functional or morphological subclasses of spinothalamic tract neurons. The presently proposed experiments will determine the anatomical distribution of opioid receptors at a resolution not possible before. Knowledge obtained from these experiments will aid in our understanding of basic opioid mechanisms, and as such will further our understanding of mechanisms related to the modulation of nociceptive and painful information in both the normal and pathologic states.